The present invention relates to a heatable pipe for storing or conveying a liquid or gaseous medium that is to be heated, wherein at least two current leads are embedded as electrodes in a layer of an electrically conductive moulding composition. The present invention also relates to a method for producing the heatable pipe.
A need which frequently arises when transporting liquid or gaseous media through a pipe is to maintain the temperature of the medium above a defined minimum temperature. Pipes of this kind are typically constructed from plastics materials; they must be able to be heated in a way which is adapted on the one hand to the medium and on the other hand to the pipe material. Examples thereof are lines for diesel fuel, where the flocculation of paraffin constituents in winter temperatures must be prevented; lines for fuel cell systems; and selective catalytic reduction (SCR) lines.
Diesel vehicles possess a catalytic converter which by means of selective catalytic reduction (SCR) uses an aqueous urea solution to cause denoxing of the exhaust gases, to lower the emissions of nitrogen oxides. However, the aqueous urea solution that is used, and which the industry consistently refers to as AdBlue®, freezes at temperatures of −11° C. and below. At these temperatures, therefore, the transport of the urea solution from the reservoir container to the catalytic converter is no longer ensured; consequently, the denoxing of the exhaust gas is no longer possible either. To prevent the AdBlue® freezing even at low temperatures, the lines must be heatable and must ensure thawing of the AdBlue® within a period of around 10 minutes.
Various methods for heating such lines have been employed. In one method standard resistance heaters are wound around the fluid pipe (WO 2009/052849). This arrangement, however, is susceptible to faults; moreover, the resistance of the wire winding must be adapted to the length of the line and to the ambient and installation conditions.
An elegant option for heating such a line is described in WO 2006/097765, WO 2006/090182, DE 39 00 821 C1 and EP 0 306 638 A1. The arrangement in each of these descriptions involves a multi-layer pipe containing two leads, which run along the pipe and are embedded, offset from one another by 180°, in a conductive polymer layer. The flow of current from one lead to the other produces heating in the conductive layer. This arrangement has considerable technical and economic advantages over the simple resistance heating. However, it does require the direct embedding of the leads in the plastics material and hence electrical contact between matrix and lead. The precise introduction of the stranded cord or wire into this layer, though, is very difficult, and becomes more and more problematic as diameters reduce. Furthermore, lines of this kind exhibit a deterioration in flexibility. The risk exists, moreover, of the contact between the conductive moulding composition and the leads undergoing change in the course of thermoforming, in the course of installation of the line in a vehicle, for example, with narrow bending radii (flexural load), on multiple freezing and re-thawing (low-temperature deformation), and in prolonged service. A consequence of such change is a change in the passage of electrons from the wire or stranded cord to the conductive moulding composition, and this, of course, has an adverse effect on the heatability of the line.
Better line flexibility is obtained with spiral winding of the electrodes. EP 0 312 204 A2 discloses a heatable line where two electrodes are wound spirally around the pipe and are embedded in an electrically conducting layer. The embedding is brought about by first coating the electrodes with the conductive moulding composition; the electrodes are subsequently wound around the inner pipe and then clad with the conductive moulding composition using a crosshead die. In this way, the contact resistance between the electrodes and the conductive layer is minimized. The process of embedding, however, is complicated. Coating of the electrodes requires a readily flowing, conductive moulding composition, whereas the moulding composition for the cladding must be applied as a relatively thick layer. EP 0 312 204 A2 gives the surface of the conductive cladding layer a smooth cylinder shape; the figures therein also show that the lead wires do not show through externally.
The object of the present invention is that of avoiding the disadvantages of the prior art and in particular of providing a line pipe with which on the one hand there is effective and durable attachment between the electrodes and the electrically conductive polymer matrix, with low contact resistance as a result of this, and on the other hand the position of the electrodes is easily detectable, allowing them to be contacted at the desired points via a coupling and fed with current.